1. Field of the Invention
The present invention relates to an improvement in a tributary interface of a digital transmission apparatus, and particularly to a tributary interface of the SDH transmission apparatus for realizing the ISDN.
2. Description of the Related Art
The ISDN (Integrated Services Digital Network) is provided, as a real transmission system, by a time-division multiplexing known as SDH (Synchronized Digital Hierarchy). In such a conventional SDH transmission apparatus, time slot interchanging is carried out by a tributary interface.
In a conventional time slot interchange unit, a redundant configuration, in which tributary interfaces of the number requested from a user are constructed so that each working interface and each protection interface are made to be a pair, was adopted.
FIG. 1 is a block diagram showing a schematic configuration of a time slot interchange unit and a tributary interface of a conventional SDH apparatus. In addition, for simplifying the explanation, an example is shown in FIG. 1 in which one tributary interface unit is connected to the time slot interchange unit.
In FIG. 1, reference numeral 10 designates a time slot interchange unit. The unit 10 is connected to a portion between a line interface 11 of a group 1 side of the high order group transmission system and a line interface 12 of a group 2 side of the high order group transmission system.
Reference numeral 1 designates a tributary interface unit connected to the time slot interchange unit 10, in which a working tributary interface 1W and a protection tributary interface 1P are built in.
In a conventional interface unit, usually, only the working tributary interface 1W is operated, which is connected to the time slot interchange unit 10. That is, the controlling overhead portion of a signal inputted to the working tributary interface 1W from the transmission system through the time slot interchange unit 10, is inputted to an overhead interface 2 to be used for synchronous control or the like. Other data portion of the signal, after the signal of the necessary channel has been extracted, is outputted to a terminal unit (not shown), and a signal outputted to the tributary interface unit 1 from the terminal unit is outputted after being multiplexed to predetermined channels, or a channel having no relation to the tributary interface unit 1 is sent out again to the transmission system intact from the time slot interchange unit 10.
Such conventional time slot interchange unit is not suitable for a situation which occurs when a user requests to increase the number of the tributary interfaces for the purpose of changing the specification or expanding the system. In reality, there is no countermeasure other than to newly design an equipment in which the requested number of interfaces would be incorporated, and therefor considerable time for developing and cost have been required to do so.